The present invention relates to a correction method for an image reading system, and more specifically, to a method of white plate correction or shading correction to minimize the width of a reference white plate in an image reading system.
The electrical industry has been developed continuously as one of the most important industries in the twentieth century. With the fast progressing in various kinds of computing and processing system, lots of electrical devices including computers, communication devices, and consumer electronics are employed in our daily life. In recent years, the application of image processing apparatus, in combining with the raising processing and storage ability of the computing system or the processors, plays an vital role in image reading, processing, and transferring usage.
Generally, most of the image reading systems can be classified as two categories. The first category is a so-called stationary reading device category. In the first category of image reading systems, the image reading device or image reading head is kept stationary during the document reading process, and the document scanning operation is achieved through moving the originals bypassing the front end of the image reading device or the image reading head. The design is mostly seen on facsimile machines and sheet-feed scanners. The simplified design provides a image reading system with less structural parts and minimize the occupied volume of the whole system.
In contrast to the stationary reading device category, the second category is called as the moving reading device or dynamic reading device category. In the second category of image reading systems, the image reading device or image reading head is driven to move in parallel, or namely in a line-by-line scanning way, in front of the stationary objects which are scanned during the image reading process. The scanning operation is completed by moving the image reading device or image reading head through the whole reading area, such as an original sheet or an object being scanned. The design is generally employed in high-quality scanning systems like typical flat-bed scanning systems, in order to provide raised image quality by the stability in the scanning operation.
In either category of the scanning systems, a line or array of image sensors are provided to read the image with the illumination of a light source. In general, the light source of a fluorescent lamp, a single or more light emitting diodes (LED), or other light emitting device is employed to provide the scanned object with essential illumination and reflect the image on the object to the image sensors.
In the aforementioned scanning systems, the uniformity in the light source and the sensibility of the image sensors is a major basis for providing high quality and accurate image scanning. However, it is known that the uniformity of the both the sensors and light source is hardly provided in the mass-manufactured industrial products. Besides, the uniformity in the light source and the sensibility of the image sensors, especially the distribution and strength in the light source through out the scanning line, may also varied with each scanning operation and the extension of operating time under the factors such as temperature, operating scheme, and other factors.
For solving the uniformity problems and provide scanned with improved accuracy, most of the scanning systems has the design of performing a correction step every time the system starts up, or even before every scanning of originals. In general, a white plate is provided as a reference plate, which is scanned in the correction step, and the brightness of the plate is served as a reference value, or generally the defined most bright level, of the image scanned later. By providing each of the scanned pixel with the same reference level of the white plate, the non-uniformity in the light distribution and the sensibility of the image sensors can be corrected.
Without limiting the scope of the present invention, a flat-bed scanning system is taken as an illustrative example to describe the design of the white plate. FIG. 1 is an illustrative view from the underside of a glass plate in a flat-bed scanning system of the typical scanner design. The scanning area 10 occupies the major part of the scanner and the originals can be read through the transparent scanning area 10 of mostly highly-transparent glass. The white plate 12 is placed before the scanning area 10 and can also covered by the same transparent glass plate.
In the set up period or before the scanning operation, a image reading device, of which the scanned area 14 is indicated in the figure, is moved through the white plate 12 and generate image signal from the white plate 12 to be utilized as a reference level for white plate correction, or so-called shading correction by skilled artisans.
In the prior art designs, a certain width of the white plate 12, for example, two hundred pixels, must be presented for providing a sufficient reference basis in the white plate correction. In addition, an additional width of the white plate 12 is also needed to compensate the misalignment in the position or the angle of the white plate 12, which is frequently happened in the manufacturing process of scanning systems, as indicated in FIG. 1, or in some occasions to compensate the small deviation or misalignment in the opening of the image reading head. As an example, for a typical scanning system having a resolution of 300 dots per inch (DPI), the additional width to compensate the manufacturing tolerance can be about 0.85 centimeter, or about the length of 100 pixels.
Therefore, to cover up the manufacturing deviation and provide the white plate correction step with sufficient width of the white plate 12, a total width of 300 pixels, as an example, may be needed for the scanning system. In a scanning system with a resolution of 300 dots per inch (DPI), the minimize width of the white plate 12 is about 1 inch, or namely 2.54 centimeters. In the conventional flat-bed scanning system, a white plate of 2.54 centimeters in width is of reasonable value in terms of the space it occupied. However, with the downsizing of the scanning system, the width of the white plate 12 has become an unresolved issue in occupying too much space of the scanning system as well as requiring too much glass plate area.
Besides the problem of occupying too much area by the white plate, another problem might be encountered. If the misalignment, or namely the skew problem, of the white plate 12 is so severe, some regions uncovered by the skewed white plate may be scanned in the correction scanning. The incorrect level on brightness, when employed as the reference level of correction, causes the failure of correction and also results in the damage of image reading quality from level-misinterpretation.
The present invention proposes a method of white plate correction to minimize the width of a reference white plate in an image reading system. The correction method provides the same accuracy and quality with conventional white plate correction while at the same time reduces the white plate width, glass area, scanner volume without influencing manufacturing tolerance. Having the proposed approach, the skewed problem of damaging image scanning quality in the prior art is also eliminated.
The correction method for an image reading system includes the major steps of: providing a reference plate for a reference level correction; reading through the image of the reference plate with line-by-line scanning of a scanning line, the scanning line having a plurality of image sensors arranged inline; picking up at least one peak value of scanned image of each of the image sensors; and correcting each of the image sensors employing corresponding the at least one peak value as a reference level.
In the preferred embodiments, the reference plate is a white plate for performing a white plate correction. In some embodiments of scanning system with different designs and correction approaches, the correction step may include the steps of: duplicating the at least one peak value to a plurality of reference values; and correcting each of the image sensors employing corresponding the reference values of each of the image sensor as reference levels.